[PATCH] t6030: explicitly test for bisection cleanup

[PATCH] t6030: explicitly test for bisection cleanup

This is not an improvement in the test coverage but it helps in making
it explicit as to know what exactly is the error as other tests are
focussed on testing other things but they do indirectly test for this.

---
I faced this problem while converting `bisect_clean_state` and the tests
where showing breakages but it wasn't clear as to where exactly are they
breaking. This will patch will help in that. Also I tested the test
coverage of the test suite before this patch and it covers this (I did
this by purposely changing names of files in git-bisect.sh and running
the test suite).